when you have a firmware password set on your Mac is not possible. Instead of hearing the usual OS X boot chime, you’ll be redirected to Recovery Mode.

In this case, you first need to disable the Mac firmware password from within Recovery Mode –> Utilities –> Firmware Password Utility. Only at that point you will be able to reset the PRAM.

It’s a good idea to enable firmware password once again after you finish troubleshooting your Mac.